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Determining the evolutionary potential of a gene

B G Hall¹, H S Malik

¹Biology Department, University of Rochester, New York 14627. drbh@uhura.cc.rochester.edu

Molecular Biology and Evolution

|August 27, 1998

Summary

The evolved beta-galactosidase (ebgA) gene in Escherichia coli can evolve new functions. Its evolutionary potential is limited to two specific amino acid replacements, suggesting a deep selective valley.

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Area of Science:

Evolutionary biology
Molecular evolution
Enzymology

Background:

Gene sequences contain information for both current functions and the evolution of new functions.
The wild-type ebgA gene encodes a nearly inactive beta-galactosidase but has the potential to evolve activity sufficient to replace lacZ for growth on specific sugars.

Purpose of the Study:

To investigate the evolutionary potential of the Ebg enzyme.
To understand the constraints on the evolution of new enzymatic functions.

Main Methods:

Comparative analysis of Ebg beta-galactosidase with homologous enzymes from diverse bacterial and eukaryotic species.
Examination of active-site residues and their evolutionary conservation.

Main Results:

Ebg beta-galactosidase belongs to an ancient clade diverging from lacZ over 2 billion years ago.
Ebg differs from other clade members at only 2 of 15 active-site residues.
The two mutations required for full Ebg activity align it with its clade members.

Conclusions:

The evolutionary trajectory of Ebg is constrained by a deep selective valley, limiting its potential to only two specific amino acid replacements.
These two replacements may represent the only acceptable amino acids at those positions or intermediates that are highly deleterious.

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